It's the defense's job to investigate the witness and show that the testimony is false, misleading, or mistaken.
It's certainly their job to cast doubt on it in some way but what happened to the burden of proof is on the claimant?
Yes, someone claiming something is a form of evidence but the burden of proof is on them to show the claim is correct.
From what I understand a witness's statement is admissible in court if it's direct but not if it's hearsay.
So "I saw Tom storming the Capitol Building" is admissible, "I heard someone say they saw Tom storming the Capitol Building" is not.
I've lost track of what you're arguing here. So claims are evidence. OK. So what?
Not all evidence is created equal. That's why Trump's cohorts kept failing in court. There were loads of people making claims but a lot was hearsay or people making vague claims they couldn't substantiate.